Determining the presence of IP multicast routers

ABSTRACT

A method for determining the presence of the multicast routers using existing protocols is disclosed. The method includes setting a time interval between a first set of query messages to each of a plurality of routers to a number greater than a querier timeout period used by the routers. The querier timeout period transitions each of the routers into a querier. The presence of each of the routers is revealed by sending a second set of query messages. Membership report messages are then sent to the routers.

BACKGROUND

[0001] This invention relates to determining the presence of Internet Protocol (IP) multicast routers, and more particularly to determining the presence of the routers using Internet Group Management Protocol (IGMP) query messages.

[0002] Network systems such as the Internet are often configured as point-to-point or unicast systems. In these systems, a message is converted into a series of addressed packets that are routed from a source node through a plurality of routers to a destination node. In many communication protocols, the packet includes a header that contains the addresses of the source and the destination nodes. The header may also contain a sequence number that specifies the packet's order in the message.

[0003] In general, these systems do not have the capability to broadcast a message from a source node to all the other nodes in the network because such a capability is rarely of much use and could easily overload the network. However, there are situations where it is desirable for one node to communicate with some subset of all the nodes. For example, multi-party conferencing capability analogous to that found in the public telephone system and broadcasting to a limited number of nodes are of considerable interest to users of packet-switched networks. To satisfy such demands, packets destined for several recipients have been encapsulated in a unicast packet and forwarded from a source to a point in a network where the packets have been replicated and forwarded on to all desired recipients. This technique is known as Internet Protocol (IP) multicast tunneling.

[0004] Recently, routers have become available which may route the multicast addresses provided for in communication protocols such as Transmission Control Protocol/Internet Protocol (TCP/IP) using a similar technique referred to as IP multicasting. A multicast address is essentially an address for a group of host computers that have indicated the desire to participate in that group. Thus, a multicast packet may be routed from a source node through a plurality of multicast routers to one or more devices receiving the multicast packets. From there the packet is distributed to all the host computers in the multicast group. Therefore, the IP multicasting conserves bandwidth by forcing the network to do packet replication only when necessary. This offers an attractive alternative to unicast transmission for the delivery of network ticker tapes, live stock quotes, multiparty video-conferencing, and other related applications.

[0005] In order to further reduce the data traffic and increase the bandwidth of the network, the source node should avoid routing the multicast packets to ports where there are no receiver nodes or routers. However under current protocols, a means for detecting the presence or absence of the multicast routers is lacking. Thus, the Internet Engineering Task Force (IETF) is currently developing a special protocol that allows determination of the presence of the IP multicast routers.

DESCRIPTION OF DRAWINGS

[0006] These and other features and advantages of the invention will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings.

[0007]FIG. 1 shows an example of an IGMP protocol operation.

[0008]FIG. 2 illustrates an IGMP pruning switch system according to one embodiment of the present disclosure.

[0009]FIG. 3 shows a state diagram for switch ports in accordance with an embodiment.

[0010]FIG. 4 shows another embodiment of an IGMP switch system having two connected switches.

[0011]FIG. 5 is a flowchart of a method for determining the presence of the multicast routers using existing protocols.

DETAILED DESCRIPTION

[0012] In recognition of the above-described difficulties, the present disclosure, in one embodiment, describes a method for determining the presence of the multicast routers using existing protocols. The method uses Internet Group Management Protocol (IGMP) query messages in conjunction with IGMP pruning. This method provides advantages over using the IETF proposed special protocol to detect the presence of the multicast routers. The advantages include not having to reconfigure routers or switch ports for implementation of this method.

[0013] The IGMP runs between hosts and their immediately neighboring multicast routers. Thus, the IGMP may be used to identify which multicast group addresses have active receivers. The mechanisms of the protocol allow a host to inform its local router that it wishes to receive transmissions addressed to a specific multicast group. Also, routers periodically query the local area network (LAN) to determine if known group members are still active. If there is more than one router on the LAN performing IP multicasting, one of the routers is elected “querier” and assumes the responsibility of querying the LAN for group members.

[0014] Based on the group membership information learned from the IGMP, a router is able to determine which, if any, multicast traffic needs to be forwarded to each of its sub-networks. Multicast routers use this information, in conjunction with a multicast routing protocol, to support IP multicasting across the Internet.

[0015] According to the IGMP specification, multicast routers, which have been elected as “queriers,” periodically transmit query messages as shown in FIG. 1. The messages are transmitted to determine which host groups have members on their directly attached networks. Therefore, if all multicast routers can be elected to serve as queriers during a determination period, the presence or absence of multicast routers may be determined when the query messages are received.

[0016] In the illustrated embodiment, a multicast router (M1 or M2) may be elected as a querier if no report messages have been received for a pre-defined querier interval. In one case, the default interval is set at about 255 seconds. Thus, the multicast router, in this case, assumes that no receivers are present. The multicast router, which is elected to be the querier, then sends query messages at every query interval. The default query interval may be set at 125 seconds.

[0017] When a host receives the query message, it responds with a report message for each host group to which it belongs. In order to avoid a flurry of report messages, each host starts a randomly chosen report delay timer for each of its group membership. If during the delay period, another report is heard for the same group, the local host resets its timer to a new random value. Otherwise, the host transmits a report message to the reported group address, causing all other members of the group to reset their report message timer. This procedure guarantees that report messages are spread out over a period of time and that report traffic is minimized for each group.

[0018] In the illustrated embodiment of FIG. 1, M1 and M2 are multicast routers. M1 has the lowest IP address and has been elected as a querier. R1 and R2 are receiver hosts. Therefore, upon reception of a general query message, the hosts set a random timer and the host with the smallest time will respond. Host S sends data to the multicast address but does not participate in the IGMP protocol.

[0019]FIG. 2 illustrates an IGMP pruning switch system 200 according to one embodiment of the present disclosure. The system 200 is adapted to determine the multicast routers present on the switch port using query messages transmitted by the routers. Thus, the objective is to avoid sending multicast data to ports on which there are no receivers or routers.

[0020] In the illustrated embodiment, the system 200 includes at least three states in which the IP multicast traffic is managed. The states are Discovery (D), Host (H), and Router (R). According to FIG. 2, the switch port connected to the sender is in Discovery state. Ports connected to multicast routers M1 and M2 are in Router state. Ports connected to receiver hosts R1 and R2 are in Host state.

[0021] In the Discovery state, each port determines whether the port is a host or router port. During the Discovery state, a pre-defined interval between a set of query messages is set to a number greater than the querier interval. For example, the interval of 300 seconds may be selected. Thus, by holding back query messages for longer period than the timeout used by the routers, the correct state may be determined. The switch generates Query3 and sends it to the host ports.

[0022] Once the determination is made, the membership report messages are only sent to ports in Router state. This forces all hosts to send membership report messages in Host state. Membership query messages are generated by the switch and sent only to host ports. This forces all routers to assume querier state, and send membership queries.

[0023] A state diagram for switch ports is shown in FIG. 3. Initially, the port is in Discovery state. Received packets or various timeout events cause state changes. Note that the port will end in Router state if both Queries and Report/Leave massages are received. Note also that two connected switches running this state machine will go to Router state on the connecting ports.

[0024] An example with two connected switches is shown in FIG. 4. The ports connecting the switches are in the Router state. This ensures that reports and data traffic are forwarded on the link between the switches.

[0025]FIG. 5 is a flowchart of a method for determining the presence of the multicast routers using existing protocols. The method includes setting a time interval between a first set of query messages to each of a plurality of routers to a number greater than a querier timeout period used by the routers at 500. The querier timeout period transitions each of the routers into a querier. Each of the routers reveals its presence by sending a second set of query messages at 502. At 504, the membership report messages are sent to the routers.

[0026] While specific embodiments of the invention have been illustrated and described, other embodiments and variations are possible. For example, although the illustrated embodiments show only two connected IGMP switches, any number of switches may be connected in the Router state.

[0027] All these are intended to be encompassed by the following claims. 

What is claimed is:
 1. A method, comprising: setting a time interval between a first set of query messages to each of a plurality of routers to a number greater than a querier timeout period used by said plurality of routers, where said querier timeout period transitions each of said plurality of routers into a querier; revealing presence of said each of said plurality of routers by sending a second set of query messages; and sending membership report messages to said plurality of routers.
 2. The method of claim 1, wherein said querier timeout period is about 255 seconds.
 3. The method of claim 2, wherein said time interval is longer than 255 seconds.
 4. The method of claim 2, wherein said time interval is about 300 seconds.
 5. The method of claim 1, wherein said second set of query messages includes queries to determine which host groups have members on directly attached networks.
 6. The method of claim 1, wherein said report messages include a report from each host group.
 7. The method of claim 6, wherein said sending includes selecting a host from said each host group to send said report.
 8. The method of claim 7, wherein said selected host starts a randomly chosen report delay timer.
 9. The method of claim 8, wherein said randomly chosen report delay timer is reset to a new random value when another report is heard from same host group.
 10. An IGMP switch system, comprising: a plurality of routers to route Internet Protocol (IP) data, said plurality of routers also generating query messages; a plurality of hosts to send and receive said IP data, said plurality of hosts also generating report messages; and an IGMP pruning switch having a plurality of switch ports, said switch ports to provide interfacing of said plurality of hosts and routers, where said IGMP pruning switch provides query messages and report messages to be transferred in such states as to allow determination of presence of said plurality of routers and hosts.
 11. The system of claim 10, wherein said pruning switch allows said plurality of hosts to issue report messages in a host state.
 12. The system of claim 10, wherein said pruning switch allows said plurality of routers to issue query messages in a router state.
 13. The system of claim 10, wherein said pruning switch determines whether each switch port is a host or router port in a discovery state by setting a time interval between a set of query messages to a number greater than a querier time out period so that said each switch port reveals its state.
 14. An IGMP switch system, comprising: a plurality of routers to route Internet Protocol (IP) data, said plurality of routers also generating query messages; a plurality of hosts to send receive said IP data, said plurality of hosts also generating report messages; and a plurality of IGMP pruning switches, each switch having a plurality of switch ports, said switch ports to provide interfacing of said plurality of hosts and routers, where said IGMP pruning switch provides query messages and report messages to be transferred in such states as to allow determination of presence of said plurality of routers and hosts.
 15. The system of claim 14, wherein said plurality fo IGMP pruning switches are connected to each other, and connecting ports are in a router state.
 16. An apparatus comprising a machine-readable storage medium having executable instructions that enable the machine to: set a time interval between a first set of query messages to a number greater than a querier timeout period used by said plurality of routers, where said querier timeout period transitions each of said plurality of routers into a querier; reveal presence of said each of said plurality of routers by sending a second set of query messages; and second send membership report messages to said plurality of routers.
 17. The apparatus of claim 16, wherein said querier timeout period is about 255 seconds. 